In semiconductor devices, the development of high crystalline structural perfection is necessary for achieving high performance in both electronic and optoelectronic devices. Commercial success of silicon (Si) complementary-metal-oxide semiconductor (CMOS) devices, and quantum mechanical devices based on epitaxial thin film deposition of gallium-arsenide (GaAs) is fundamentally due to availability of low cost and extremely high crystalline perfection bulk substrates of Si and GaAs, respectively.
Less traditional semiconductor materials such as those formed of group III-nitride (III-N) materials offer the potential for expansion in device functionality, particularly for high power electronics and optoelectronic devices such as ultraviolet (UV) light emitting diodes (LEDs) and lasers. Semiconductor optoelectronic devices convert electrical energy into optical energy by taking advantage of the interaction of electrical energy with the semiconductor's crystal structure which has a specific electronic energy configuration known as the electronic band structure. Semiconductor light sources generate light using semiconductor junctions comprising at least a p-type semiconductor region and an n-type semiconductor region. The p-type semiconductor region is designed to be a source of holes, whereas the n-type region is a source of electrons. Under the appropriate external electrical bias, electrons and holes are injected from their respective sources towards an intrinsic layer, which serves as an electron-hole-recombination region. Group III-nitride material is generally the most mature wide bandgap semiconductor material and is widely used in UV and visible LEDs in the wavelength range of 250 to 600 nm.
While epitaxial deposition of sequential and multiple composition thin films of III-N is well-known in the art, the commercial success of III-N devices is still limited by the available substrates that can be used. The commercially available substrates compatible for III-N epitaxial deposition suffer disadvantages such as high production cost, available wafer diameter, lack of crystal structure match to III-N, and lack of surface quality provided by the substrate for epitaxy.
Recently, bulk freestanding and low crystalline defect density GaN and AlN substrates have become commercially available for application in power electronics and optoelectronic device markets. Electronic and optoelectronic III-N devices formed on these high quality bulk GaN and AlN substrates using thin film deposition techniques show high performance, primarily due to the much closer match in crystal structure between epitaxial film and the substrate and the resulting low defect density attained within the active layer films. Unfortunately, the cost of these GaN and AlN substrates hinder widespread use. Additionally, limited available wafer diameters and presence of impurities hinder their use in deep UV.
Due to the lack of availability of large native substrates with suitable transparency at UV wavelengths, III-N epigrowth is typically performed on sapphire, silicon (Si) or silicon carbide (SiC) substrates, all of which have high lattice mismatch to III-N materials such as aluminum nitride (AlN) and aluminum-gallium-nitride (AlGaN). Growth of device stack epilayers on a dissimilar substrate material generates a large number of threading dislocations (e.g., on the order of 1010 cm−2) in the epistack. Threading dislocations are defects which propagate vertically through an epifilm, usually originating at the interface between the substrate and epifilm. The three types of threading dislocations are screw, edge, and mixed. Screw dislocations propagate in a helical pattern perpendicular to the stress direction. Edge dislocations occur when an extra plane of atoms is present within a crystal structure. Mixed defects are intermediate between screw- and edge-type. Threading dislocation density (TDD) in the intrinsic layer of a semiconductor LED device is an important factor in determining the internal quantum efficiency (IQE) and therefore light output intensity of LEDs, as they provide non-radiative recombination sites. The presence of defects also affects other operational parameters, such as leakage currents and lifetime of the device.
Typical solutions for reducing the TDD include growth of very thick layers, lateral overgrowth on patterned substrates, growth on very small lattice mismatched native substrates of AlN or GaN, use of superlattice strain buffer layers, and insertions of a variety of very thin interlayers or masking structures such as SixNy and Ti. Due to difficulties associated with the growth kinetics—such as difficult lateral growth of AlN and long growth times—most of these options are not suitable for molecular beam epitaxy (MBE) grown materials.